A medicated plaster may comprise a backing material coated with an adhesive, and a reservoir of medicine adhering to a central part of the backing. The backing material should be impermeable to the medicine. When the periphery of the backing adheres to the skin, the reservoir of medicine is contacted with the skin, and the medicine permeates the skin.
The reservoir may comprise a hydrophilic or lipophilic polymer matrix in which the medicine is dispersed. The medicated polymer can be molded into a disc of determined surface area and thickness. The release of the medicine is governed by its diffusivity in the matrix and absorbability by the skin. The problem is to design a therapeutic system in which the skin penetration rate is determined by the rate at which the medicine is delivered to the skin surface and not the inherent permeability of the skin. See Patent Specifications U.S. 4,031,894 and EP 33615A2.